It is known that some underwater structures, for example, underwater pipes, are subject to cracks. Cracks in underwater pipes can be detected with conventional apparatus that travels inside of the pipes and that can detect the cracks ultrasonically. Such techniques generally require that the flow of material in the pipes be temporarily suspended while the inspection of the pipes is ongoing.
The ability to image, with sonar systems, the outside of underwater pipes and other comparatively smooth surfaced underwater objects, particularly to identify irregularities or cracks, is difficult, in part because the wavelength used in typical imaging sonar is typically on the order of several millimeters to several centimeters. It is not possible to distinguish features on the target surface which are at, or smaller than, the wavelength of the sonar system. Cracks can be less than one millimeter across.
Optical techniques, either LIDAR or conventional photography, have an advantage because the wavelength of light is much shorter than the wavelength of sound, and thus, the optical system can achieve higher image resolution than a conventional sonar system. However, optical techniques also have disadvantages because they cannot be used effectively in turbid (murky) waters. Also, even at short range, optical techniques tend to require the use of high energy illumination which is not well suited for being use on a small autonomous underwater vehicle (AUV) with limited internal power capability.
Active sonar systems transmit a signal into the water, and receive echoes from targets or surfaces in the water. The targets can include, but are not limited to, submarines, torpedoes, tethered mines, bottom mines, cables, bottom features such as rocks, outcrops, pipelines, and the bottom surface itself.
The acoustic receiver portion of an active sonar system can have a single receive element or an array of receive elements arranged in a receive array, typically formed as a line array. With this arrangement, signals provided by the receive elements can be added to provide a receive beam having a beamwidth inversely proportional to a length of the receive array. Relative time delays or relative phase shifts can be introduced to the signals provided by the receive elements to steer the receive beams about the receive array.
Sidescan imaging sonar generally uses a linear transmit array to generate sound in a narrow horizontal beam of acoustic energy, which is directed obliquely towards the seabed. Images are formed by moving this narrow transmit beam through the water and receiving echoes or scattered sound resulting from the transmitted sound.
With regard to across track image resolution, for a sidescan imaging sonar that uses a frequency modulated transmit waveform, across track resolution is determined by a bandwidth of a transmit pulse. For a sidescan imaging sonar that uses a single frequency transmit waveform transmitted as a pulse, across track resolution is determined by a length of the pulse (still related to a bandwidth of the transmission).
With regard to along track resolution, for a sidescan imaging sonar, above described receive beamforming techniques can be used to focus energy received as echoes or backscatter from the seabed or target object, resulting in an improved along track resolution. The extent to which the along track resolution can be improved with receive beamforming techniques has a fixed limit based upon the along track aperture of the receive array. Conventional sidescan sonar is limited in resolution to an angular resolution (in radians) defined by the wavelength of the sidescan sonar divided by the aperture of the receive array.
Where the receive array is a horizontal receive array disposed in a line, an azimuth width of a receive beam (related to an along track resolution) is inversely proportional to the length of the array in wavelengths. Therefore, high along track resolution can be achieved either by lengthening the receive array or by increasing the frequency of operation. However, physical array length is often limited by a size of a platform to which the receive array is attached. Furthermore, high acoustic frequencies attenuate rapidly in the water, preventing acoustic propagation to long ranges. As a result, conventional active sonar systems are limited in performance by receive array length constraints and by acoustic frequency constraints.
Synthetic aperture techniques can improve the along track resolution beyond that described above for conventional receive beamforming.
Synthetic aperture sonar (SAS) is a known processing technique that uses an array of underwater receive elements, usually disposed in a line array, to receive and combine sound resulting from successive sound transmissions or pings as the line array moves through the water.
SAS improves along track resolution of a moving active sonar system by coherently combining receive signals associated with more than one acoustic transmission or ping to synthesize a longer effective receive array, i.e., a “virtual array.” With SAS processing, Nyquist sampling constraints require that the receive array advance (move along its axis) by no more than half the physical length of the receive array between successive pings. It is known that more rapid movement of the receive array results in formation of grating lobes. A variety of SAS algorithms are known.
SAS requires knowledge of, i.e. an estimate of, a spatial track (i.e., position or motion estimate) of the receive array with time, in order to be able to accurately coherently combine receive signals from the receive array. It will be understood that a spatial “track” can be associated with six degrees of freedom of motion of the receive array: three displacements and three rotations. For a flexible receive array, for example, a towed line array, spatial estimates can be difficult. For a rigid receive array, for example, as may be disposed upon an autonomous underwater vehicle (AUV), the estimates may be less difficult.
To further improve an image, an autofocus technique can automatically adjust the track estimate based on an ability of the adjustment to improve the quality of a resulting SAS image.
SAS is conventionally performed at relatively low acoustic frequencies and over large distances between the SAS and the underwater target from which echoes are received. SAS tends to be used merely to detect the presence of underwater objects and structures, e.g., submarines, mines. As described above, it is known that relatively low frequencies can provide only coarse image resolution of detected objects, and thus, conventional SAS sonar systems and methods do not lend themselves to imaging of small irregularities on an underwater surface, e.g., cracks on an underwater pipe.
It would be desirable to provide a system and method for inspecting pipes and other underwater surfaces for cracks and the like, without stopping flow of material within the pipes.